Effects of oxide coating and selected cations on nitrate reduction by iron metal

Under anoxic conditions, zerovalent iron (Fe-0) reduces nitrate to ammonium and magnetite (Fe3O4) is produced at near-neutral pH. Nitrate removal was most rapid at low pH (2-4); however, the formation of a black oxide film at pH 5 to 8 temporarily halted or slowed the reaction unless the system was augmented with Fe2+, Cu2+, or Al3+. Bathing the corroding Fe-0 in a Fe2+ solution greatly enhanced nitrate reduction at near-neutral pH and coincided with the formation of a black precipitate. X-ray diffractometry and scanning electron microscopy confirmed that both the black precipitate and black oxide coating on the iron surface were magnetite. In this system, ferrous iron was determined to be a partial contributor to nitrate removal, but nitrate reduction was not observed in the absence of Fe-0. Nitrate removal was also enhanced by augmenting the Fe-0-H2O system with Fe3+, Cu2+, or Al3+ but not Ca2+, Mg2+, or Zn2+. Our research indicates that a magnetite coating is not a hindrance to nitrate reduction by Fe-0, provided sufficient aqueous Fe2+ is present in the system.